The present invention relates to a torque superimposition device for a hybrid drive and to a method for operating the hybrid drive comprising the torque superimposition device according to the invention. The hybrid drive is particularly suited for use in a passenger motor vehicle.
Numerous hybrid drives are known in the prior art. In most instances, an electric motor is arranged in what is known as a hybrid parallel architecture in such a way that the drive torque of the electric motor can be connected to the output shaft of the internal combustion engine. The sum moment of internal combustion engine and electric motor is then conducted through the transmission, and thereafter to the driven output shaft of the vehicle. In recuperation mode, the moments flow with the reverse sign. Because the electric motor and internal combustion engine are coupled upstream of the transmission, the two propulsion systems, these being the internal combustion engine and the electric motor, are each operated in the same gear. In the hybrid vehicles presently available in the market, the selected gear is generally dependent on the respective resulting operating point of the internal combustion engine. The electric motor is often operated at a less than optimal operating point. This results in drawbacks, either in terms of power or from an energy point of view.
Other approaches provide for a power splitting of the drive torque. Two electric motors are required for this purpose, of which one is operated as a generator and the other as a motor. Power splitting results in energy losses.
The advantages and disadvantages of these hybrid architectures are known in the prior art.
A further approach is described in DE 10 2010 063 092 A1. A hybrid drive comprising a torque superimposition device is described here, which comprises a first gearing device associated with the electric motor, and a second gearing device associated with the internal combustion engine. Each gearing device has at least two alternatively selectable gear ratio steps. The moments of the electric motor and of the internal combustion engine are added on the output side downstream of the two gearing devices and supplied to the differential of the vehicle via the torque output of the torque superimposition device. In recuperation mode, the torque flows with the reverse sign. This topology allows the propulsion systems to be shifted separately from each other. This topology, however, is still not optimal with respect to the tractive force or tractive force development or tractive force continuity, in particular when aggregate power is needed during the respective shifting processes. In addition, the topology is complex in terms of the installation space.
It is the object of the invention to create a torque superimposition device that takes the above-described disadvantages better into account. In particular, a torque superposition device is to be created that allows operation of both the internal combustion engine and of the electric motor at the respective optimal operating point of the same. Moreover, the torque superimposition device is to be designed in such a way that the disadvantages in terms of power or energy known from the prior art are eliminated, and in particular that unnecessary energy losses are avoided. Moreover, the torque superimposition device is to be optimized with respect to the necessary installation space. In particular, a method for operating such a hybrid drive comprising a torque superimposition device according to the invention having high energy efficiency is also to be provided.
This object is achieved according to the invention by the torque superimposition device and the method disclosed and claimed herein. Further advantageous embodiments of the invention can be found in the dependent claims.
The torque superimposition device according to the invention is intended primarily for use in a hybrid drive for motor vehicles, in particular for use in passenger motor vehicles. For this purpose, the hybrid drive comprises an internal combustion engine, an electric motor, and the torque superimposition device designed according to the invention, wherein a torque of the electric motor can be superimposed on a torque of the internal combustion engine by means of the torque superimposition device. The torque superimposition device is moreover connected on the output side to an output of the vehicle.
The torque superimposition device implemented according to the invention comprises a first and a second torque input, a torque output, and a first gearing device and a second gearing device. The electric motor can be coupled to the first torque input in a torque-proof manner, and the internal combustion engine can be coupled to the second torque input in a torque-proof manner. The first torque input is connected to the first gearing device in a torque-proof manner, and the second torque input is connected to the second gearing device in a torque-proof manner. The gearing devices are coupled in each case on the output side to the torque output of the torque superimposition device in a torque-proof manner.
One idea of the invention that should be highlighted is therefore that, in principle, each propulsion system—these being the electric motor and the internal combustion engine—has a dedicated gearing device available. The drive torques of the propulsion systems are not added up until the output of the gearing devices. This summation, on the output side, of the drive torques in a positive torque flow direction downstream of the respective gearing devices constitutes a very essential functional difference compared to the hybrid gearboxes known from the prior art, in which the summation typically takes place upstream of the gearing device. The output-side summation makes it possible to select the respective shifted gear ratio steps of the propulsion systems independently of each other, and therefore in a way that is optimized for the particular propulsion system.
The first gearing device has at least two alternatively selectable gear ratio steps, and the second gearing device has at least one alternatively selectable gear ratio step.
According to the invention, the first gearing device comprises at least one planetary gear set to implement the at least two alternatively selectable gear ratio steps, while the second gearing device comprises at least one spur gear set to implement the at least one selectable gear ratio step.
It has been shown that, by combining a planetary gear set on the side of the first gearing device with at least one spur gear set on the side of the second gearing device, very favorable designs of the torque superimposition device in terms of the installation space can be found or achieved. Furthermore, the two alternatively selectable gear ratio steps of the first gearing device can be implemented very favorably from an efficiency point of view because a planetary gear set provides favorable conditions for this. In this way, the electric motor-based torque can be transmitted from the electric motor to the output with optimal efficiency, which results in an increase in the electric range of the vehicle equipped with the torque superimposition device implemented according to the invention. Due to the comparatively low energy storage densities of electric storage devices, for example of known Li-Ion storage devices, high efficiency in the electrical path of hybrid drives is particularly advantageous. It has been shown that it is advantageous to design the first gear ratio step of the first gearing device, serving as the first electric motor-based gear, in a driving power-oriented manner such that a desired high starting acceleration can be implemented by the electric motor, while the second gear ratio step of the first gearing device, serving as the second electric motor-based gear, should advantageously be designed such that the maximum speed of the vehicle can be implemented by the electric motor. Depending on the electric motor, it may also be advantageous to select a longer design for the second electric motor-based gear so as to open up additional energy efficiency potential via optimized operating points of the electric motor.
The planetary gear set can be either a single planetary gear set or an expanded planetary gear set, such as a Ravigneaux planetary gear set.
Moreover, the design of the second gearing device comprising at least one spur gear set allows the gear ratio of the spur gear set to be easily adapted within the scope of the layout of the drive train. It is therefore possible, with low modification complexity with respect to the gear ratio of the spur gear set of the second gearing device, to design the hybrid drive for other vehicle weights, for higher or lower power of the internal combustion engine, or for higher or lower maximum speeds of the vehicle. The person skilled in the art is familiar with such designs.
When the second gear ratio step is configured with only one selectable spur gear set, the gear ratio of the spur gear set will be expediently selected so that the maximum speed of the vehicle can be implemented with this gear ratio.
According to an advantageous refinement of the invention, each of the two gearing devices has at least two alternatively selectable gear ratio steps. The second gear ratio step of the second gearing device can then expediently be designed as an energy efficiency gear, which is to say having a longer gear ratio than the first gear ratio step of the second gearing device. This results in higher energy efficiency of the hybrid drive when operation takes place by means of the internal combustion engine.
According to a further refinement of the invention, however, it may also be advantageous to design the second gear ratio step of the second gearing device to have a shorter gear ratio than the first gear ratio step of the second gearing device, either within the meaning of a driving gear or even a starting gear.
So as to implement the at least two alternatively selectable gear ratio steps, the second gearing device expediently comprises at least two spur gear sets for this purpose, which in a preferred embodiment of the invention are arranged behind each other on an input shaft connected to the second torque input. Shifting elements for the selectability of the gear ratio step can therefore be designed uniformly within the second gearing device.
According to a particularly advantageous embodiment of the invention, the first gearing device comprises exactly one planetary gear set, wherein preferably exactly two gear ratio steps can alternatively be selected. It has been shown that two gear ratio steps are usually sufficient to be able to select the operating point of the electric motor in the hybrid drive designed according to the invention in a way that is specific to the propulsion system, both with respect to optimal driving power and with respect to high energy efficiency, under all driving conditions. At the same time, the installation space requirement is very low for only one planetary gear set of the first gearing device, so that the entire torque superimposition device can have a very compact design. This is particularly advantageous for integrating the hybrid drive into compact cars having very limited installation space with usually transversely arranged propulsion systems.
It is advantageous to expediently design the planetary gear set with at least one power-shift friction brake and/or friction clutch for the selection of the gear ratio steps. For example, the sun gear of the planetary gear set comprising a friction brake or friction clutch can be implemented so as to be breakable with respect to the housing, so that the movement of the planet carrier to the ring gear forms a gear ratio step when the sun gear is braked. Due to the power-shiftability of friction brakes and/or friction clutches, it is possible to implement very comfortable and fast shifting sequences without interruption of torque flow.
In a particularly advantageous refinement of the invention, the planetary gear set can be configured as a block to implement one of the gear ratio steps of the at least two alternatively selectable gear ratio steps. In a planetary gear set having a block configuration, the entire gear set rotates as a block, whereby toothing losses within the planetary gear set are eliminated. This is very desirable from an efficiency point of view. Accordingly, the gear ratio step that is to be selected to have a block configuration is that which, during operation of the vehicle, is engaged with the highest energy throughput in driving operation. It has been shown that this is typically the second, longer gear ratio step. However, depending on design, it is also possible for the first gear ratio step to be implemented by means of a planetary gear set having a block configuration.
According to a further expedient embodiment of the invention, the electric machine can be coupled to a ring gear of the planetary gear set in a torque-proof manner, a sun gear of the planetary gear set can be coupled to a housing of the torque superimposition device in a stationary manner, preferably by means of the power-shift friction brake, and a planet carrier of the planetary gear set is coupled to the torque output in a torque-proof manner so as to implement a further gear ratio step of the at least two alternatively selectable gear ratio steps. With an appropriately designed stationary gear ratio of the planetary gear set, such an interconnection of the planetary gear set can be used to implement gear ratios of up to 2 in a very compact construction with high moment load ability by braking the sun gear with respect to the housing.
The second gearing device moreover expediently comprises at least one form-locked and/or frictionally engaged clutch for the selectability of the at least one spur gear set designed as a gear ratio step. Advantageously, all gear ratio steps are designed with form-locked and/or frictionally engaged clutches. For example, dog clutches can be used as form-locked clutches. Dog clutches are generally not power-shift clutches, so that such shifting from one internal combustion engine-based gear ratio step or gear of the second gearing device into another internal combustion engine-based gear ratio step or gear must take place either with assistance from the electric motor, or when a tractive force reduction results on the output side during the shifting process. When the internal combustion engine-based power is small compared to the electric motor-based power, this will be less relevant; however, at higher internal combustion engine-based power levels, the driver may perceive this to be undesirable, in particular during full-load maneuvers that are carried out with aggregate power. In these cases, technologically more complex frictionally engaged clutches are preferable, which allow moment overlap during the shifting process from one gear ratio step to the next.
In many cases, it is advantageous if the at least one clutch is configured with a synchronizer element. Suitable synchronizer elements, such as single-cone to triple-cone synchronizers, are known in the prior art. Synchronizer elements can be used to passively reduce, which is to say without active engine intervention, rotational speed differentials between the shaft and the gear to be shifted in a short period of time sufficiently for the shifting process to be completed, for example by engagement of the dog clutches. Advantageously all clutches are implemented with synchronizer elements.
In an advantageous refinement of the invention, a fixed gearing element is arranged between the electric motor and the planetary gear set, preferably a spur gear set having a first spur gear facing the electric motor and a second spur gear facing the planetary gear set. The fixed gearing element preferably has a gear ratio between 1.5 and 4, particularly preferably a gear ratio between 1.5 and 3, most particularly preferably a gear ratio of 1.5 to 2. In this way, pre-gearing of the electric motor speed can be accomplished, so that the planetary gear set sees considerably lower rotational speeds. This is in particular advantageous for high-speed electric motors because in this way the rotational speed-dependent losses in bearings or gear meshing events are reduced in the planetary gear set.
To implement one or two further alternatively selectable gear ratio steps, a shaft of the second gearing device, which is connected to the second torque input, can particularly advantageously be connected to the first spur gear of the fixed gearing element designed as the spur gear set in an alternatively selectable and torque-proof manner. This essentially represents a “co-use” of the electric motor-based gear ratio steps that are available in the torque superimposition device as internal combustion engine-based gear ratio steps. The gear ratio steps of the propulsion systems, however, can then no longer be freely selected with respect to each other. Care must therefore be taken to ensure that these gears are selected appropriately for each other with respect to the two propulsion systems. This is the case when starting, for example, where the two propulsion systems are operated with a short gear ratio step, which is to say a high gear ratio. The same applies to an intermediate driving power gear, which is to be implemented in particular for maneuvers at high load or full load and aggregate power of the propulsion systems.
Another advantageous embodiment of the invention provides for the shaft of the second gearing device, which is connected to the second torque input, and a shaft of the first gearing device, which is connected to the first torque input, to be arranged aligned with each other, and for the end of the shaft of the second gearing device, which faces away from the second torque input, to be mounted in an end of the shaft of the first gearing device which is designed as a hollow shaft, the shaft being connected to the first torque input. This results in very compact shapes, however with a series arrangement of the propulsion systems comprising the interposed torque superimposition device, designed as a transmission.
So as to allow an alternatively selectable torque-proof connection of the shaft to the fixed gearing element, a form-locked and/or frictionally engaged clutch is advantageously arranged on the shaft for this purpose, and the clutch is preferably implemented with a synchronizer element.
The invention also covers a hybrid drive designed according to the invention, in particular for use in a passenger motor vehicle, comprising an internal combustion engine, an electric motor, and a torque superimposition device as described above. For this purpose, the internal combustion engine preferably has a power between 40 and 150 kW, particularly preferably a power between 50 and 100 kW. This involves continuous power that the internal combustion engine is able to provide. The electric motor expediently has a power of 30 to 60 kW, and the peak power corresponding to 2 to 3 times the continuous output. The invention is particularly suited for drive systems in which the peak power of the electric motor corresponds at least to the continuous power of the internal combustion engine, or is considerably above that. Short-term power reserves are in particular advantageous to provide assistance during shifting processes of the internal combustion engine.
The method according to the invention for operating a hybrid drive of a motor vehicle, in particular a passenger motor vehicle, wherein the hybrid drive comprises a torque superimposition device according to one of the preceding claims, provides for the method to comprise a normal hybrid operating mode having a first and a second control range.
The first control range covers a vehicle speed of zero up to an electric driving speed limit in the normal hybrid driving mode, and the second control range covers the electric driving speed limit in the normal hybrid driving mode up to the maximum speed of the vehicle. In the first control range, the drive torque of the motor vehicle is generated only by means of the electric motor, and in the second control range, the drive torque of the motor vehicle is generated in hybrid operation by a cooperation of the internal combustion engine and the electric motor. The internal combustion engine is operated independently of the load requirement of the driver input in the range of the optimal efficiency of the internal combustion engine.
According to an advantageous refinement, the internal combustion engine is operated briefly at loads above the range of optimal efficiency of the internal combustion engine only in a special driving power mode.
So as to ensure optimal utilization of the battery charge state within the meaning of the greatest possible electric driving component, but also to ensure optimal and energy-efficient use of the stored energy (electric, chemical), the method advantageously comprises a first special hybrid operating mode, which replaces the normal hybrid operating mode as a function of the battery charge state. This first special hybrid driving mode is activated when the battery charge state drops below a first limit value. Upon activation of the first special hybrid driving mode, the electric driving speed limit is simultaneously set to a value lower than the original value.
According to a preferred embodiment, the method moreover comprises a second special hybrid operating mode, which replaces the normal hybrid operating mode as a function of the battery charge state. This second special hybrid operating mode is activated when the battery charge state exceeds a second limit value. Optionally, a charging rate by the internal combustion engine that on average is positive can be used as an additional condition for activating the second special hybrid operating mode. Upon activation of the second special hybrid operating mode, the electric driving speed limit is simultaneously set to a value higher than the original value and/or the operation of the internal combustion engine is controlled to a load below the range of optimal efficiency of the internal combustion engine in such a way that the charging rate by the internal combustion engine decreases on average, is equal to zero, or even turns negative.
These measures are used to select the respective operating mode as a function of the battery charge state. The respective operating states are thus reproducible and comprehensible for the driver. Unexpected operating behavior of the drive system is therefore eliminated.
As a refinement, the above-described battery charge state-dependent selection of the operating mode can be supplemented and/or overridden with anticipatory functions. For example, using anticipatory functions (such as via a driver input and/or information from the navigation system), it is possible to deliberately increase (such as when entering Low-Emission Zones) or decrease (such as when charging options exist at the destination) the battery charge state. The battery charge can be controlled via the effective charging rate by the internal combustion engine and/or by changing the electric driving speed limit.
So as to convey to the driver a distinct electric driving experience or satisfy regulations, the method can moreover comprise an electric driving operating mode which the driver can optionally select and in which the drive torque of the motor vehicle is generated only by means of the electric motor across the entire speed range. Again, it is possible to control the selection of the electric driving operating mode using anticipatory functions (such as by driver input and/or information from the navigation system). By means of example, an electric driving operating mode can be selected in this way in city zones or zones defined by regulations, thereby forcing emission-free operation.
The torque superimposition unit according to the invention in combination with the method for operation according to the invention therefore allows efficient use of both types of energy to an extent as great as possible. The decision as to which type of energy is used is primarily made based on system efficiencies and energy efficiency reasons. By separating the base load (primarily implemented by the internal combustion engine) and dynamics for acceleration processes (primarily implemented by the electric motor), in conjunction with an increase in the load level for an efficient operation of the internal combustion engine and for the generation of the electric energy for boost processes, the present invention is particularly suited for internal combustion engines that have been optimized in terms of efficiency at the expense of responsiveness, dynamics, and the operating range.
Further details, features and advantages of the invention will be apparent from the following description of one exemplary embodiment based on the drawings. In the drawings:
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.